This invention relates to dialysis machines of the type used in artificial kidney systems, and more particularly, to a blood pressure alarm system for use therein.
In a dialysis machine water and concentrate are mixed to provide dialysis solution which is delivered to a dialyzer through which both blood and dialysis solution flow on opposite sides of a semipermeable membrane. Waste products from the blood pass through the membrane into the dialysis solution for disposal. Normally dialysis takes approximately 4-6 hours.
Dialysis machines are equipped with both arterial and venous blood pressure alarm systems for activating an alarm and for deactivating a blood pump in the extracorporeal blood circuit in the event that the blood pressure in the blood circuit exceeds or falls below predetermined values. This is sometimes referred to as an alarm window. Proper monitoring of both the arterial and venous pressure is important since failure or errors in monitoring can result in blood loss from the patient.
One alarm monitoring system provides for alarm conditions when the blood pressure varies by more than .+-.50 mm/Hg from an adjustable and manually set pressure point. The pressure selector is a knob having an indicating arrow which is set with respect to pressure indicating markings on a face plate. A meter is provided which displays the actual pressure but not the selected pressure. A comparator is provided to compare actual pressure against the set point .+-.50 mm/Hg. This system had disadvantages in that: (1) the nurse could err in setting the reference point; (2) the face plate/knob relationship could be off which would result in an erroneously selected reference point; and (3) the machine characteristics could vary which would result in an erroneous reference point.
In an effort to overcome these problems, an unmarked plunger-type knob was provided which cooperates with the meter for the setting of the reference point. With the knob in the out position, the meter displays actual blood pressure and, when pushed to the in position, the meter is engaged and the reference point can be selected against the meter scale. The alarm is still set .+-.50 mm/Hg above and below the reference point. With this system the errors due to knob mounting and machine error are eliminated and the internal pressure transducer and alarm set knob referenced against the same meter. However, this system is inconvenient to operate since the knob has to be pushed in and out to set while watching the meter. Furthermore, the variability about the reference point could not be controlled.
In a third generation machine, provision is made to set the reference point using the pressure produced when the dialysis machine is operating and the patient's condition has stabilized. By moving a slide switch from a set-up mode to an operate mode, the reference point is set into the machine. This eliminates the need for the plunger-type knob, and a second slide control is provided by which the variability about the reference point can be adjusted between .+-.10 and .+-.100 mm/Hg. In this system a memory is provided which stores the reference point. The memory is essentially a capacitor, and the charge on the capacitor is updated every 5 minutes during dialysis by comparison against the actual blood pressure at that point in time, so long as no alarm condition had been met. The blood pressure at 5 minutes, 10 minutes, etc., can be different than the desired reference point. It should be noted that the variability is set against the memory point. Thus, changes in the charge on the capacitor could result in changes in the alarm conditions which would be undesirable.
It is therefore an object of this invention to provide a memory system for use in a blood pressure alarm system in a dialysis machine, wherein the alarm conditions remain fixed relative to their initial settings with time.
This and other objects will become apparent from the following description and appended claims.